You probably changed your USB mode in the settings menu. Files added under a given mode can only be seen when connected in that mode. So if you originally added files under Auto/MTP mode and the changed to MSC mode, the Fuze will still see the files but your computer no longer will. Try switching the USB mode.
It depends on what USB mode the unit is set to. If the USB mode is set to Auto Detect or MTP mode, it will appear as a Music device with it’s own hierarchal folder structure. If set to MSC mode, it appears as a removable drive with it’s own folder structure. Although the player can play and catalog music that is added in either mode, when you are in MSC mode, you will only see content that was added in MSC mode when connected to the computer. If in MTP mode, you will only see content you added in MTP mode.
The only exception to this rule is how the MicroSD card is treated. It seems that the same directory structure on the MicroSD card exists regardless of what mode you use, so you can see any content added in whatever mode on the SD card.
Thanks so much for this thread. I thought Fuze was a goner when my files disappeared (music, and many, many voice recordings, also). I don’t even remember changing the USB setting, but when I changed it to Auto Detect, my files miraculously reappeared.